If you planned it out ahead of time (greenscreens, shot correctly) you could create 2 layers of the video, time remap it and just mask the area you want in slow motion.
If it's not shot well though, you will have very visible seams between the the stuff moving at regular speed and the slow motion, it will look like 2 different shots composited on top of each other.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.